The present invention relates to a method for producing a semiconductor device using a semiconductor having crystallinity.
A thin film transistor (TFT) and a thin film diode (TFD) using a crystalline silicon thin film semiconductor are well know. The TFT and TFD are constructed by forming a thin film semiconductor on an insulting substrate or an insulating surface formed on a semiconductor substrate. The TFT is used in various integrated circuits. In particular, the TFT may be used as a switching element to be formed in each pixel of an active matrix type liquid crystal display device, a driver element formed in a peripheral circuit portion, and an element of a three dimensional integrated circuit.
To obtain crystalline silicon film used in such elements, a method for heating an amorphous silicon film at 600xc2x0 C. or higher is well known. In this method, a crystalline silicon film is obtained by solid-phase-growing a silicon film having an amorphous state (an amorphous silicon film). Also, to obtain a crystalline silicon film, a method for melting a silicon film by irradiating a laser light or an intense light corresponding to the laser light and then cooling the silicon film for solidification is well known.
However, a silicon film obtained by these methods is polycrystalline. Therefore, since there are grain boundaries in the silicon film, characteristics of a TFT having such silicon film are inferior to that of a TFT having a single crystalline silicon film. In addition, since a generating position of grain boundaries cannot be adjusted, variations in characteristics of TFTs to be formed are extremely large.
The object of the present invention is to solve the above problem, and to obtain a silicon film having a high crystallinity by adjusting progress of crystallization and a generating position of grain boundaries.
In the present invention, a silicon film is melted by irradiating a pulse laser light or an intense light (coherent light or incoherent light) corresponding to the pulse laser light, that is, a light energy which an output is varied largely. After irradiating the light energy, solidification is adjusted by providing two dimensional differences in a cooling speed of a silicon film. On portion of silicon film in which a cooling sped is speed is fast is crystallized early by solidification. Another portion if the silicon film in which the cooling speed is slow produces a state which crystallization is not made. In such state, since the crystallization portion becomes nucleus, a crystal growth extends from one portion to the another portion. As a result, obtained crystalline silicon has an extremely high crystallinity. When a suitable condition is set, a substantially single crystalline silicon film in which grain boundaries are not present in an area of 10 xcexcm to 1 mm square can be obtained.
In the present invention, in order to obtain the above cooling speed distribution, a film having a high thermal (heat) conductivity material such as aluminum nitride, boron nitride, and diamond may be formed selectively. These materials may be crystalline or amorphous.
A silicon film is formed directly or indirectly on or under the high thermal conductivity material film. When the high thermal conductivity material film is formed on the silicon film, this film can be removed after crystallization processing. Therefore, degree of freedom in a device structure is increased. Also, when diamond is selected as the high thermal conductivity material, it can be removed by oxidation easily using hydrogen plasma processing-or the like.
In the present invention, a silicon film may be amorphous, microcrystalline, or polycrystalline. The silicon film is selected in accordance with absorbency of the laser light or the like. When the silicon film having crystallinity is not completely melted, remaining portions of the silicon film become nucleus, so that another crystal growth different from the crystal growth in the present invention may produce.
To further effectively obtain a silicon film having a high crystallinity in the present invention, the silicon film may be heated at 400xc2x0 C. or higher when irradiating a laser light or the like. Since this heating decreases generally a cooling speed after a laser irradiation, it is effective in progress of crystal growth.
A substrate may be thin. Since the substrate also operates as a heat sink, heat supplied to the silicon film by irradiating a laser light or the like is absorbed immediately by the substrate. When the substrate is thick, a thermal capacity is large, and it tends to enhance this relationship. On the other hand, when the substrate is thin and is surrounded by a thermal insulating material, heat diffusion is prevented. This also decreases generally a cooling speed after a laser irradiation, it is effective in progress of crystal growth.
When there is a problem in a mechanical stress by thinning the whole substrate, the same effect is obtained by thinning only necessary the portion of the substrate. For example, a hole is formed in only portion of the substrate corresponding to a region in which a thin film transistor (TFT) is formed.
It is further effective when a method for heating a silicon film combines with a method for thinning a substrate.
When a TFT is formed using the above obtained silicon film having a high crystallinity, a portion having a single crystalline state or a substantially single crystalline state may be formed as a channel forming region of the TFT. The substantially single crystalline state does not represent a complete crystalline state but a state wherein through it is necessary to neutralize dangling bonds in a crystalline region by adding hydrogen or halogen element, a crystal orientation other than a main crystal orientation is 1% or less by a structure analysis method such as an X-ray diffraction method and a Raman scattering spectroscopic analysis (a state that crystal orientation is aligned largely). Since such single crystalline state or substantially single crystal state is not obtained in a region in which a high thermal conductivity material film is formed, essentially a channel forming region is formed in another region other than the region in which the high thermal conductivity material film is formed.
In the present invention, a crystal nucleus always produces at a desired position in a cooling process after a laser irradiation. A crystal growth stably occurs from the desired position. Therefore, the crystallization process has extremely high reproducibility. Also, a variation in crystallinity due to a change of laser energy is sufficiently small, so that a yield of a thin film semiconductor device to be obtained is extremely high.